1. Field of the Invention
This invention relates to an article of manufacture based on a structurally defined laminate. More particularly, this invention relates to a unitary, composite, flexible laminated article having electrically conductive means disposed therein, and in which layers of the laminate are bonded to each other. This invention also relates to the use of such a laminated article as a heating device and to a method for preparing the article.
2. Description of the Prior Art
A number of low-temperature electric radiant heaters utilizing resistor wires, etched metal foils and electrically conductive films have been described in the prior art. For the most part, they operate at temperatures below about 250.degree. F. These heaters are used in a variety of applications, including personal comfort heating, agribusiness and industrial processes.
For example, radiant heating panels have been fabricated by embedding resistive wires in an insulating substrate, such as gypsum board, or by applying the wires directly to the insulating substrate. A variety of types of etched aluminum and other metal foil heating elements have also been proposed. For instance, a commercially available unit comprises a metal foil laid between two layers of clear polyester film.
Resistive wire heaters and etched metal heaters suffer a major deficiency in that all of the current is usually carried by a single continuous conductor. A break anywhere in the electrically conductive path renders the entire heater inoperable. Additionally, since the resistive wires operate at relatively high temperatures, such as up to 400.degree. F., they must be covered and insulated so as not to char or weaken the substrate with which they are in contact. The substrate interposes an additional insulative layer between the heating element and the intended receptor and thereby reduces the effectiveness of heat transmission. They also require high voltages, such as 208 to 288 volts AC. In addition, non-oxidized metals have a low emissivity, which reduces their effectiveness in converting electrical energy into radiant heat.
Metallic powders, transparent vapor-deposited metals or metal oxides, and conductive carbon black or graphite have been proposed for use as electrically conductive films in radiant heaters. These materials may be deposited between layers of silicone rubber, polyester film or asbestos-like paper, which are then encased in a painted, galvanized metal shell.
Devices using electroconductive films have not proved entirely satisfactory. When a metal is used in the electrically conductive material, the metal is frequently of high cost. This is particularly true of thick-film heaters based on noble metals. Heaters based on aluminum, tin oxide, indium oxide and similar materials are also costly, limited in size and restricted to low temperature use. Conductive metal powder coatings eventually develop oxides on their particulate surfaces, which raises their resistance to such a high extent that they are rendered inoperative. The electrical resistivity of electrically conductive films comprised mainly of carbon may vary with age and the conditions of use. Moreover, insulation of radiant heaters based on electroconductive films is usually difficult because it is necessary to cut into the conductive film and through the dielectric insulating cover. This raises a variety of potential problems ranging from corrosion of exposed connections, with a resulting increase in resistance, to the possibility of mechanical damage or corrosion to the interior of the heating panel.
Accordingly, there exists a need in the art for an electrically conductive laminated article useful as a radiant heater. The device should be useful at relatively low operating temperatures, such as about 250.degree. F. or less. The article should be of a unitary structure making it unnecessary to cut or otherwise open or separate the lamina in order to install the article in its intended location. It should also be possible to make electrical connections to the article without damaging or destroying the external laminae. The article should be waterproof and substantially impermeable to moisture and harmful gases and vapors. It should have a high emissivity to ensure efficient transmission of radiant heat. Ideally, the article should be capable of producing infra-red radiation in a band width that would be subject to minimal atmospheric absorption or interference yet achieve maximum absorption by a human body. The device should exhibit a stable resistivity. Moreover, the resistivity should be controllable, reproducible and predictable. The article should be fire-resistant and safe if accidentally punctured. It should also be capable of being operated at 110 volts AC or lower voltages. The article should have good aesthetic qualities and be capable of being produced at relatively high speed using conventional equipment.